Energy storage is important for many applications from hybrid vehicles to off-peak electric power to rotating machinery. A flywheel offers the combination of high energy density and high power density not attainable with other energy storage medium. Numerous applications utilize flywheels, typically either for purely storing energy or for the purpose of minimizing the angular velocity fluctuation of a shaft. Examples of energy storage applications include flywheel hybrid vehicles, uninterrupted power supplies, cyclic alternative energy sources such as wind turbines, and space power systems. Applications utilizing a flywheel for smoothing angular velocity fluctuations include an internal combustion engine, industrial machinery such as camshafts, and AC generators. In the design of a system with a conventional flywheel used to minimize changes in angular velocity, the flywheel is sized for an allowable coefficient of fluctuation, defined as the change in angular velocity during a cycle divided by the average angular velocity. To achieve a low coefficient of fluctuation, a flywheel with a large moment of inertia is required.
From the equation for the kinetic energy storage of a flywheel,E=½Iω2,
where I is the mass moment of inertia and ω is the angular velocity, it can be noted that a change in energy can be accommodated with a change in the angular velocity, as in a conventional flywheel, or through a change in the moment of inertia. Utilizing a variable inertia flywheel theoretically enables a zero coefficient of fluctuation with a smaller and lighter flywheel. Alternatively, a variable inertia flywheel can eliminate the need for a continuously variable transmission between the flywheel and the load. The inertia of a flywheel can be changed in multiple ways including moving mechanical masses, allowing the flywheel material to strain, and the like.
In many situations, it is desirable to store energy at a constant angular velocity. One of the earliest variable inertia flywheels was the flyball governor by James Watt. Other moving mass variable inertia flywheels include designs using sliding masses on tracks and band-type variable inertia flywheels (BVIF). The BVIF uses a thin metal band wrapped between an inner and outer drum. By changing the angular difference between the inner and outer hubs, the wrappings of the band are transferred between the two drums. To create this angular difference between the two hubs, attempts to recirculate power from the flywheel using a planetary gear train were made.
Instead of moving fixed masses through mechanical means, another method of creating a variable inertia flywheel is to allow the centripetal acceleration of the flywheel create strain in the material. Due to the non-linear stress-strain relationship of specific elastomers, approximately 80% of the stored energy in the system can be extracted at a nearly constant angular velocity. However, due to the limited strength of elastomeric materials with the required stress-strain behavior, the energy density of the flywheel is significantly limited.
Variable inertia flywheels may be applied in various applications, such as hydraulic energy storage applications. Hydraulic energy storage is important to numerous applications including hydraulic hybrid vehicles and alternative energy sources such as wind turbines. Hydraulic energy is typically stored in a hydraulic accumulator, which is typically a pressure vessel containing a gas that is compressed by the addition of hydraulic oil to the pressure vessel. The energy storage density of hydraulic accumulators is significantly lower than other energy storage mediums. The consequence of this for applications such as hydraulic hybrid vehicles is a concession in the energy storage capacity based on packaging and weight considerations. The limited energy storage is a barrier to technologies such as “plug-in” hydraulic hybrid vehicles that can operate for a considerable distance solely on energy storage.
Previous research on improving the energy density of hydraulic accumulators has primarily focused on isothermalizing the compression and expansion of the gas in the accumulator and adding foam or fine metallic strands to the gas volume. These approaches have provided incremental increases in the energy density of hydraulic accumulators, yet the energy density is still orders of magnitude lower than competing technologies.